Description of problem:

After LOOP_SET_FD/LOOP_CLR_FD combo loop device's queue gets request
handler which is persistent.

After, say

        mount -t iso9660 /dev/loop0 /mnt        # sic

this request handler is called directly with
a) ->lo_state being Lo_unbound
b) ->lo_pending being zero

Error path in loop_make_request() completes ->lo_bh_done completion
which is persistent as well.

Now, let's start worker thread as usual. It'll set ->lo_pending to 1,
don't wait for completion because it was already completed (brokenly),
and will not get out of infinite loop because of ->lo_pending. Loop
device doesn't have bios at this point and triggers BUG_ON.

So, don't complete ->lo_bh_done when loop device isn't setup fully.

How reproducible: always
Steps to Reproduce:

#!/bin/sh -x
ISO=1.iso
mount -o loop $ISO /mnt
umount /mnt
mount -t iso9660 /dev/loop0 /mnt        # sic
mount -o loop $ISO /mnt
  
Actual results: BUG_ON(!bio); in loop_thread triggers